The preparation of the oligomeric phosphonic anhydrides of the formula (II) (OPA) by condensation with a suitable assistant (EP-B-0 527 442) and the use of these OPAs to form amide bonds (DE-A-38 39 379) is already known.
However, to form amide bonds, it is necessary here in accordance with the state of the art to use excesses of OPA because, in the product prepared according to EP 0 527 442, phosphonic anhydrides of chain length P20-P200 are prepared and the composition of the OPA is thus not known precisely. Resulting costs can be avoided by virtue of the cyclic propanephosphonic anhydrides of the formula (III) (CPA) prepared by the process according to the invention.
Phosphonic acids are degraded by microorganisms typically to the phosphate, which is ecologically problematic (eutrophication). The use of the cyclic phosphonic anhydrides (CPA), which is reduced because it is stoichiometric, can prevent this. This achieves a further advantage by the preparation and the use of compounds of the formula (III) compared to the known preparation processes and applications, as described in EP-A-0 527 442 and EP-A-3 839 379.
The selected preparation of CPAs in the laboratory has already succeeded (H. Wissmann, H. J. Kleiner, Angew. Chem. Int. Ed. 1980, 19, 133 [129]). The possibility of a distillation of the CPAs is mentioned here, but it is an additional purifying distillation of CPAs already present as crude products. Moreover, in this process, the CPAs are prepared using phosphonyl dichlorides.

The HCl formed in this preparation process poses many problems in an industrial application of this synthesis route as a result of its corrosive and toxic properties. An additional fact is that the product comprises chloride, which can restrict the possible uses of the CPAs.
With regard to the aforementioned disadvantages of the known processes, in which a product mixture is obtained in undefined composition, large amounts of waste occur, corrosive and toxic gases form or the product comprises chloride, there is a need to provide an improved process which does not have all of these disadvantages.